slothbear / rails_admin_array_filter

Playground for adding array filters to rails_admin

rails_admin + PostgreSQL arrays

rails_admin does not support database-specific functions. The maintainers suggest creating a rails_admin plug-in for such features.

I'm new to both rails_admin and PostgreSQL arrays, so I'm exploring how they can work together.

Make it work, make it right, make it fast.

I have a Rails application that requires an admin be able to delete a selection of records via a tag. Each record has one or more tags in a PostgreSQL array. If you're not familiar with the feature, check out this great post by Bernardo Chaves at the Platformatec blog: Rails 4 and PostgreSQL Arrays

Here are the steps of my bare-bones implementation:

Configure the tags field in the rails_admin initializer:

config.model 'Bug' do
  list do
    field :tags do
      searchable true
    end
  end
end

rails_admin currently omits array fields since they didn't work well. searchable true adds the field to the interface. The tags field now also appears in the Add filter drop-down menu.

Give it a try with vanilla rails_admin

Choose the tags field from the Add filter menu, enter any value in the tags search field, then press the Refresh button. A string array looks like a simple (scalar) string to rails_admin, and the query fails:

PG::UndefinedFunction: ERROR:  function lower(text[]) does not exist
HINT:  No function matches the given name and argument types.
You might need to add explicit type casts.
: SELECT  "bugs".* FROM "bugs"
WHERE ((LOWER(bugs.tags) ILIKE '%def%'))
ORDER BY bugs.id desc LIMIT 20 OFFSET 0

rails_admin applies the LOWER function to an array, which PostgreSQL does not understand. Even if the LOWER function were removed the query would still fail as PostgreSQL does not support the ILIKE operator on an array.

Why does the query include the LOWER function? Both the PostgreSQL ILIKE operator and the more common LIKE operator are case-insensitive. Why bother lower-casing only one side of a case-insensitive match? Even if some database implemented LIKE as case-sensitive, applying LOWER to only one argument wouldn't help. Would it?

Implementing 'array contains tag'

PostgreSQL defines a set of operators for dealing with arrays. We need the contains operator, @>.

WHERE (bugs.tags @> ARRAY['system-crash'])

This finds records where the tag 'system-crash' is in the tags array. To see if I'm on the right track, let's add the array query at the easiest spot in the rails_admin SQL StatementBuilder method #build_statement_for_string_or_text.

contains_clause = ["#{@column} @> ARRAY[?]", @value]
return contains_clause if @column == "bugs.tags"

The query is formed correctly and finds the records where the tags array contains the filter text. Note that the @> operator is case-sensitive, so the new query only finds exact matches.

Preparing for production

Remaining challenges:

• Find the right place for the contains operator logic.
• Make the new array filter work for any column
• Add a filter search_operator for array contains
• Remove irrelevant search operators (e.g. starts_with, ends_with)
• Update related record counts after bulk_delete
• Can I change the label of for an "Add filter"? My custom filter doesn't select everything that matches (internal requirements).